1. Field of the Invention
The present invention relates to a semiconductor physical quantity sensor, having a beam-structure movable portion, for measuring a physical quantity, such as acceleration, yaw rate or vibration, and a method of producing such a sensor.
2. Description of the Prior Art
In general, a physical quantity sensor, such as an acceleration sensor, employs a so-called flexure beam so as to detect movement of a mass portion coupled to the beam upon application of a physical quantity to the mass portion.
There has been a demand for reduction in size and price of physical quantity sensors, such as acceleration sensors for use in automobile suspension controls, automobile air bags or the like. U.S. Pat. No. 5,465,604 or Japanese Second (examined) Patent Publication No. 6-44008 corresponding to the former discloses a differential capacitive semiconductor acceleration sensor having a polysilicon beam structure with electrodes, which may satisfy such a demand. The sensor of this type will be explained hereinbelow with reference to FIGS. 34-36. FIG. 34 is a perspective view of the sensor, FIG. 35 is a sectional view taken along line XXXV--XXXV in FIG. 34, and FIG. 36 is a sectional view taken along line XXXVI--XXXVI in FIG. 34.
Beams 132 rest above a silicon substrate 130, each extending between corresponding anchors 131 fixed on the substrate 130. A mass 133 is suspended by the beams 132, and movable electrodes 134 are projected from the mass 133. On the silicon substrate 130, a pair of fixed electrodes 135a and 135b are arranged relative to each of the movable electrodes 134. Specifically, the movable electrode 134 is interposed between the pair of fixed electrodes 135a and 135b so as to face them at opposite sides thereof. The movable electrode 134 and the corresponding fixed electrodes 135a and 135b form a pair of capacitors having capacitances C1 and C2, respectively, which serve to achieve a servo control of the movable electrode 134. The anchors 131, the beams 132, the mass 133 and the movable electrodes 134 are made of polysilicon, and the beams 132, the mass 133 and the movable electrodes 134 are disposed spacing a given distance from the surface of the silicon substrate 130. The fixed electrodes 135a and 135b are fixed on the substrate 130 at their ends through anchors 136, respectively. The foregoing members are formed on the silicon substrate 130 using a surface micro-machining technique.
An operation of the acceleration sensor thus structured for detecting acceleration will be briefly explained hereinbelow with reference to FIG. 35.
When acceleration is zero, each of the movable electrodes 134 is located at the middle point between the corresponding fixed electrodes 135a and 135b, and the capacitance C1 of the capacitor formed by the movable and fixed electrodes 134 and 135a and the capacitance C2 of the capacitor formed by the movable and fixed electrodes 134 and 135b are set equal to each other. Further, when acceleration is zero, a voltage V1 applied across the capacitor formed by the movable and fixed electrodes 134 and 135a and a voltage V2 applied across the capacitor formed by the movable and fixed electrodes 134 and 135b are also set equal to each other so that each movable electrode 134 is attracted to opposite sides, that is, toward the fixed electrodes 135a and 135b, with electrostatic forces F1 and F2 of the same magnitude, respectively. On the other hand, if acceleration is applied in a direction parallel to the surface of the substrate 130 so as to displace the movable electrode 134 relative to the fixed electrodes 135a and 135b, the capacitances C1 and C2 are caused to have different values so that the electrostatic forces F1 and F2 become unequal to each other. Then, the magnitudes of V1 and V2 are so controlled as to cause the electrostatic forces F1 and F2 to have values which displace the movable electrode 134 to the middle point between the fixed electrodes 135a and 135b to render C1 and C2 equal to each other. When C1 and C2 are rendered equal to each other, the applied acceleration and the electrostatic force (that is, a differential between F1 and F2) are balanced so that the magnitude of acceleration can be derived based on the controlled magnitudes of V1 and V2.
Fabrication of the acceleration sensor shown in FIG. 34 will be explained hereinbelow with reference to FIGS. 37 and 38. In FIG. 37, a sacrificial layer (silicon oxide film) 138 is deposited on a silicon substrate 137, and openings 139 are formed at given regions in the sacrificial layer 138. Then, a polysilicon film 140 is deposited on the sacrificial layer 138 including the openings 139 and patterned into a given shape. Further, as shown in FIG. 38, the sacrificial layer 138 is removed by etching to form an air gap 141 so that a beam structure of the polysilicon film 140 is achieved.
As described above, in the foregoing acceleration sensor shown in U.S. Pat. No. 5,465,604, the beam structure is formed of polysilicon, that is, polycrystalline silicon. However, mechanical materiality values of polycrystalline silicon are unknown so that the mechanical reliability thereof is less ensured as compared with monocrystalline silicon. Further, the beam structure is subjected to camber or warp due to internal stresses or a stress distribution generated upon formation of the polycrystalline silicon film on the silicon oxide film formed on the monocrystalline silicon substrate. This causes the fabrication of the beam structure to be difficult, causes the spring constant of the sensor to be changed, or the like.
On the other hand, the mechanical reliability may be improved by using an SOI (silicon-on-insulator) substrate and monocrystalline silicon as a beam structure. The sensor of this type will be explained hereinbelow with reference to FIGS. 39-41. FIG. 39 is a plan view of the sensor, FIG. 40 is a sectional view taken along line XXXX--XXXX in FIG. 39, and FIG. 41 is a sectional view taken along line XXXXI--XXXXI in FIG. 39.
In this acceleration sensor, a vibration mass 147 is coupled to stationary supporters 146 via flexible beams 145 so that the vibration mass 147 is allowed to displace. The vibration mass 147 is formed of monocrystalline silicon doped with P (phosphorus). The stationary supporters 146 are fixed on a monocrystalline silicon substrate 148 in an electrically insulated state. The vibration mass 147 is provided with movable electrodes 149 extending in parallel to each other. The members 145, 146, 147 and 149 constitute a beam structure 150. Fixed electrodes 151 and 152 are arranged facing the movable electrodes 149, respectively, so as to define capacitances between the movable electrodes 149 and the fixed electrodes 151 and between the movable electrodes 149 and the fixed electrodes 152. With this arrangement, if acceleration is applied in a direction parallel to the surface of the substrate 148, such as in a direction Y in FIG. 39, the movable electrodes 149 displace relative to the fixed electrodes 151 and 152 via the flexible beams 145 so that the capacitances therebetween change.
Fabrication of the acceleration sensor shown in FIG. 39 will be explained hereinbelow with reference to FIGS. 42-46. In FIG. 42, in order to form an SIMOX layer on the monocrystalline silicon substrate 148, oxygen ions (O.sup.+ or O.sub.2.sup.+) of 10.sup.16 -10.sup.18 dose/cm.sup.2 are implanted in the substrate 148 at 100 keV-1,000 keV, and the ion-implanted substrate 148 is subjected to heat treatment at 1,150.degree. C.-1,400.degree. C. Through the foregoing process, an SOI substrate having a silicon oxide layer 153 of about 400 nm in thickness and a surface silicon layer 154 of about 150 nm in thickness is formed. Then, as shown in FIG. 43, the silicon layer 154 and the silicon oxide layer 153 are partly removed through photolithography and etching. Further, as shown in FIG. 44, a monocrystalline silicon layer 155 is epitaxially deposited in thickness of 1 .mu.m-100 .mu.m (normally, 10 .mu.m-20 .mu.m). Subsequently, as shown in FIG. 45, after formation of a metal film on the monocrystalline silicon layer 155, the metal film is formed into electrodes 156 of given shapes through photolithography for connection to a measuring circuit. Further, as shown in FIG. 46, the reactive vapor phase dry etching is applied to the monocrystalline silicon layer 155 so as to form the fixed electrodes 151 and 152, the movable electrodes 149 and so on. Finally, the silicon oxide layer 153 is removed through the HF liquid phase etching so as to provide the movable beam structure 150.
However, if the servo control as performed in the acceleration sensor shown in U.S. Pat. No. 5,465,604 is performed using the SOI substrate, it is necessary that the silicon substrate is formed with wiring using an impurity diffusion layer for crossing a first fixed electrode energization line and a second fixed electrode energization line. Specifically, as shown in FIGS. 47 and 48, a silicon substrate 160 is provided with a comb-shaped movable member 157 having rod electrodes 157a extending in parallel to each other above the silicon substrate 160. On the silicon substrate 160, first fixed electrodes 158 and second fixed electrodes 159 are further arranged such that each of the first fixed electrodes 158 faces one side of the corresponding rod electrode 157a while each of the second fixed electrodes 159 faces the other side of the corresponding rod electrode 157a. Further, the second fixed electrodes 159 are connected to each other (formed into a comb-shaped electrode) on the silicon substrate 160, while the first fixed electrodes 158 are electrically connected to each other through an impurity diffusion layer 161 formed on the silicon layer 160. However, in this case, leakage current is generated at the impurity diffusion layer 161 so as to render it difficult to detect acceleration with accuracy. Particularly, the acceleration detection is liable to be influenced by the leakage current at high temperatures.
In general, the silicon oxide film is used for the insulating layer. However, in the sacrificial layer etching process for rendering the beam structure movable, it is difficult to control the etching amount in a transverse direction upon removing the silicon oxide film (sacrificial layer). Accordingly, it is difficult to control lengths of the beams so that spring constants of the beam structures may differ from each other. Specifically, it is difficult to accurately control the concentration and the temperature of an etchant at constant values, and it is also difficult to accurately control the termination of etching. Thus, when leaving portions of the silicon oxide layer (sacrificial layer) as the anchors, it is difficult to process the beams into a given shape.